1. Field of the Invention
The invention relates generally to a method and an apparatus for removal of impurities from noble gases and nitrogen. In particular, the invention relates to a method and apparatus which utilize a single purifier vessel with three temperature zones to achieve removal of contaminants below 1 part per billion (ppb). The first zone preheats in-coming gas, the second zone removes methane and other impurities, and the third zone removes hydrogen. A recuperative heat exchanger preheats in-coming gas utilizing hot gas exiting the purifier vessel.
2. Brief Description of the Prior Art
Hot getter materials have been widely used to remove impurities from noble gases and nitrogen. The getter material is typically encased in a stainless steel container maintained at a single temperature. At the temperatures at which these purifiers are operated, out gassing of hydrogen from the getter and stainless steel container may contaminate the purified gas to levels of 0.01 to more than 1 part per million (ppm). U.S. Pat. No. 5,238,469 describes a method for removal of hydrogen after other impurities are removed with hot getter materials. The impure gas is preheated to a temperature greater than 200.degree. C. (preferably greater than 300.degree. C.) in a first preheating vessel. The heated impure gas is then passed into a second vessel over a heated getter material (preferably at greater than 300.degree. C.) where impurities such as methane, water, carbon monoxide, nitrogen, oxygen, and carbon dioxide are removed. The heated gas is then cooled to less than 100.degree. C. by passing through a heat exchanger. Finally, the gas is passed through a third vessel which is filled with a sorption material with a high capacity for hydrogen. U.S. Pat. No. 5,238,469 indicates that hydrogen impurity levels in nitrogen may be reduced to less than 1 ppb using this technique. However, this technique requires use of three vessels and a heat exchanger, and requires that the gas being purified be cooled below 100.degree. C. (and preferably to 40.degree. C.) with the heat exchanger before hydrogen can be removed to less than 100 ppb.
Another prior art technique which has recently been used involves a single purifier vessel having two temperature zones. In the first higher temperature zone, a getter is used at about 350 to 400.degree. C. to remove impurities such as methane, water, carbon monoxide, oxygen, and carbon dioxide. A second, lower temperature zone (200 to 250.degree. C.) is used to remove hydrogen with a second getter suited to hydrogen removal. The impurity removal with this technique is less than 1 ppb for methane, water, carbon monoxide, oxygen, and carbon dioxide, and less than 10 ppb for hydrogen. At these levels, flow rates of only 100 SCFH can be achieved with the system described.